A liquid crystal display element has advantages over other display element in terms of thinness, lightness in weight, and low power consumption. With these advantages, the liquid crystal display element is widely used for image display devices such as a television or a video, a monitor, OA (Office Automation) equipments such as a word processor, and a personal computer.
There are conventionally well-known liquid crystal display modes for the liquid crystal display element, such as the TN (Twisted Nematic) mode using Nematic liquid crystal, the display mode using FLC (Ferroelectric Liquid Crystal) or AFLC (Anti-Ferroelectric liquid crystal), or the polymer dispersed liquid crystal display mode.
Among the liquid crystal display modes, for example, the TN mode liquid crystal display element has come into practical use. However, the TN mode liquid crystal display element has some drawbacks such as slow response, a narrow viewing angle etc. Those disadvantages are large hindrances for the TN mode to take over CRT (Cathode Ray Tube).
The mode using the FLC or AFLC allows high-speed response and wide viewing angle, but is inadequate in terms of shock resistance or temperature characteristics. Such a defect has kept the FLC or AFLC display device from wide and practical application.
The polymer dispersed liquid crystal display mode uses light scattering, and does not need a polarizer, while allowing high-luminance display. However, the polymer dispersed liquid crystal display mode has a problem in its response property upon image display. Therefore, the polymer dispersed liquid crystal display mode has few advantages over the TN mode.
In each of those liquid crystal display elements, liquid crystal molecules are aligned in a certain direction, and the viewing angle depends on an angle with respect to the liquid crystal molecules. That is, in those display modes, there is restriction in the viewing angle. Further, each of the display modes uses rotation of the liquid crystal molecules caused by electric field application, wherein the liquid crystal molecules rotate together maintaining the alignment, thus a response speed is slow. In the meantime, the mode using the FLC or AFLC is superior in response speed and viewing angle, but the mode has a problem of irreversible alignment breakdown due to external force.
Apart from the display elements using the molecule rotation caused by application of the electric field, there has been suggested a liquid crystal display element using a material whose optical isotropy changes in response to electric field application, particularly a material causing orientational polarization due to electric optical effect, or electronic polarization.
The term “electro-optic effect” indicates such a phenomenon that reflective index of a substance varies according to an external electric field, and there are two types in the electro-optic effect: (i) the Pockels effect that is proportional to the electric field, and (ii) the Kerr effect that is proportional to square of the electric field.
Substances exhibiting the Kerr effect were adopted early on for high-speed optical shutters, and have been actually used for special measuring instruments. The Kerr effect was found by J. Kerr in 1875. Well-know substances exhibiting the Kerr effect are organic liquid materials such as nitrobenzene, carbon disulfide, and the like. Apart from the optical shutter, these substances are used for, for example, high electric field strength measurement for an electric cable or the like.
Later on, research has been conducted to utilize a large Kerr constant of the liquid crystal materials for use in light modulation devices, light deflection devices, and optical integrated circuits. There has been a report of one liquid crystal compound which has a Kerr constant more than 200 times higher than that of nitrobenzene.
Under such circumstances, studies for using the Kerr effect to a display device have started. In view of the fact that the refractive index of a material exhibiting the Kerr effect is proportional to the square of electric field application, an assumed effect by use of the material exhibiting the Kerr effect as the orientational polarization attains a relatively low voltage driving than the orientational polarization made of a material having the Pockels effect. Further, with its original response property of several μ seconds to several m seconds, the substance exhibiting the Kerr effect is assumably suitable for a high-speed response display device.
Under the circumstances, for example, Documents 1, 2 and 3 detailed below propose a display element which is formed by sealing in a medium made of a liquid crystal material between a pair of substrates, and applying a voltage perpendicular or parallel to the substrates so as to induce the Kerr effect. Particularly, in the display device of Document 1, an alignment film is deposited on each of the planes of the pair of substrates in contact with the medium.
In such a display element, two polarizers with axes orthogonal to each other are provided outside the substrates, respectively, so that the medium is optically isotropic when no electric field is applied, thereby displaying black, and generates a birefringence when an electric field is applied. In this way, the transmittance of display element changes, and displays gradation. This method achieves a significantly high value of contrast in the normal substrate direction.
However, those conventional display elements have not achieved significant reduction of driving voltage; and therefore are not sufficient for practical use.
Particularly, in the display device of Document 1, both planes of the pair of substrates in contact with the medium are respectively provided with alignment films, and therefore the voltage upon application of electric field is impressed to not only the medium but also the alignment films. Therefore, due to the voltage consumed for the alignment film, the amount of voltage impressed to the medium is reduced. Consequently, the display device of Document 1 offsets the effect of reduction of driving voltage.